“It is not credible,” New York Mayor Bill de Blasio said. He added: “Our school system remains fully open.”

The LAPD chief, Charlie Beck, said he supported the decision to close the schools.

“It is very easy in hindsight to criticize a decision based on results that the decider could never have known,” Beck said. “It’s also very easy to criticize a decision when you have no responsibility for the outcome of that decision.”

De Blasio said NYPD intelligence analysts scrutinized the email: “This was a very generic piece of writing,” he said, and the threatened violence “is not plausible.”

“It’s very important not to overreact in situations like this,” de Blasio said.